Alloy for hollow cathode lamp



Dec. 30, 1969 J. w. VO LLMER 3,487,254

ALLOY FOR HOLLOW CATHODE LAMP Filed Jan. 16, 1969 INVENTOR. v JOHN w-VOLLMER United States Patent 3,487,254 ALLOY FOR HOLLOW CATHODE LAMP John W. Vollmer, Daubury, Conn., assignor to The Perkin-Elmer "Corporation, Norwalk, Conn., a corporation of New York Filed Jan. 16, 1969, Ser. No. 791,637 Int. Cl. H01j 17/04, 61/06 U.S. Cl. 313218 3 Claims ABSTRACT OF THE DISCLOSURE This invention relates to an improved hollow cathode used in a lamp, of the type especially useful as a light source in spectroscopic instruments of the atomic absorption type. More particularly this invention concerns an improved hollow cathode for a lamp, which is useful primarily for atomic absorption measurements of both tellurium and tin.

In atomic absorption spectroscopy the sample (containing one or more metals) is analyzed by determining the absorption (at a certain specific wavelength of radiation) caused by the atoms for which the analytical test is being made. Usually the metallic sample (typically in the form of a dissolved salt) is vaporized in the flame of a burner, so that the sample is atomized, and then irradiated with a light source of great intensity at least at one characteristic absorption band of the tested-for-metal. Only transmitted radiation in the region of this characteristic wavelength is then allowed to reach a detector, which therefore yields a measurement of how much absorption has occurred. The detected intensity (as compared to the original source intensity, for example) yields a quantitative measurement of the absorption and therefore the concentration of the particular metal for which the analysis is being conducted. A description of atomic absorption spectroscopy and spectrometers is contained in United States Patent No. 2,847,899, issued Aug. 19, 1958 to A. Walsh.

In. order to irradiate the sample at high intensity in the narrow absorption band, the light source itself preferably includes a relatively high concentration of the metal for which the test is being made. A typical such light source is the hollow cathode lamp, in which a cup-shaped element (including at least a substantial percentage of the metal for which the test is intended) acts as the negative electrode of the lamp. Both this hollow cathode and the positive electrode are hermetically sealed within a glass envelope in a low pressure atmosphere of an inert (noble) gas. A typical such hollow cathode lamp is shown in applicants United States Patent No. 3,361,925, issued Jan. 2, 1968. Preferably the hollow cathode assembly of the present invention is incorporated in a shielded hollow cathode lamp of the type having an open generally cylindrical, shield of insulating material substantially coaxial with the cathode, as shown, for example, at 60 in FIGURE 1 of applicants United States Patent No. 3,390,- 297, issued on June 25, 1968.

As noted in applicants said Patent No. 3,361,925, the actively radiating part of the hollow cathode should have certain desirable physical characteristics to provide high intensity spectral radiation, to have a long useful life, and to simplify manufacture. In addition to having such desirable mechanical properties and to having physical and chemical stability at all tempertures to which it is likely to be exposed in use, the actively emitting cathode material must also have certain desirable electrical characteristics. in particular, this cathode material should have an electrical resistance of a convenient value (i.e., relatively low) under the actual operating conditions of the lamp, and the resistance-versus-temperature curve (over the range from room temperature through operating temperature) should be smooth.

Pure tellurium is an extremely brittle metallic substance, which exhibits relatively high vapor pressure at the temperatures normally reached within the hollow of the cathode lamps of the type noted above. In addition to these nop-ideal physical characteristics, the electrical resistivity of tellurium is substantially higher than the low ideal value. By utilizing an alloy of tellurium, in particular tin-telluride, applicant provides a material of substantially increased ductility, reduced vapor pressure at operating temperatures, and reduced electrical resistance as compared to pure tellurium. Additionally, the melting point of this alloy is higher than that of pure tellurium, so that the hollow cathode lamp can be operated at higher currents (and therefore yield higher radiation intensity). Additionally, the use of this alloy in the interior of the hollow cathode enables a lamp to be utilized not only as a tellurium radiation source but also as a tin radiation source.

An object of the invention is the provision of an improved hollow cathode assembly for use in a lamp for emitting radiation in the characteristic spectrum of tellurium.

A similar object is the provision of such a hollow cathode assembly, in which the tellurium-containing substance has greater ductility, reduced vapor pressure at operating temperature, lower electrical resistance, and higher melting point than previously utilized pure tellurium, thereby obtaining, respectively, greater ease of manufacture, longer life at a given operating current, lessened voltage requirements for the lamp current supply, and higher practical operating currents (and therefore intensity).

Another object of the invention is the provision of an improved spectral radiation source lamp, which provides radiation not only in the characteristic spectrum of tellurium, but also in the characteristic spectrum of another metal, for example tin, so that the same lamp may be utilized in the analysis of either of such metals.

Other objects and advantages of the invention will become obvious to one skilled in the art on reading the following detailed description of an exemplary embodiment of the invention in conjunction with the accompanying drawing in which:

The sole figure i an enlarged vertical cross-section through the improved hollow cathode assembly, incorporating in its interior a tellurium alloy according to the invention.

The draWing shows a finished hollow cathode assembly according to the invention prior to its being incorporated into a lamp of the type shown, for example in the above mentioned US. Patent No. 3,361,925 or preferably of the type shown in FIGURE 1 of US. Patent No. 3,390,297. The hollow cathode assembly 20 of the instant drawing includes a conventional cathode cup or holder 22, which is preferably made of copper in the combination of the present invention. The closed (left hand in the drawing) end of the holder 22 has a narrowed portion 24, having a central narrow aperture 25 for press fitting or crimping upon the cathode pin of hollow cathode lamp (as shown in either of applicants above mentioned two patents). The wider open-cup portion (right-hand in the drawing) of 26 of the cathode holder will contain a generally cylindrical insert or slug 28, having a generally cylindrical central aperture or hollow at 29. This insert 29 comprises the material which determines the actual radiation characteristics of the lamp, and in the preferred embodiment of the invention will be made of tin-telluride, as more explicitly described hereinafter.

A major point of novelty of the present invention is the constituency of the actively emitting substance of the insert 28. Specifically by utilizing tin-telluride for the substance, an improvement is obtained in both ease of manufacture and in the characteristics of the lamp (including intensity of the radiation) for use as a source of tellurium spectral radiation. Additionally, the lamp is a highly satisfactory (in fact, improved) source for tin spectral radiation. An exemplary technique for making a hollow-cathode assembly, according to the invention is as follows.

A suitable amount (about grams being required for each insert) of tin-telluride may be made by alloying tin (48.2 percent by weight) with a chemically equivalent amount of tellurium (51.8 percent by weight) in an inert atmosphere (for example, an argon forming gas in a carbon crucible /s, of an inch internal diameter) by means of an induction heater. If a single 5 gram batch is thus prepared, a slug or insert of tin-telluride with a inch external diameter and about /2 inch long will be formed. This slug is faced off and drilled (Ma inch diameter) so as to form the central longitudinal bore or hollow 29 of the drawing to a depth of approximately 78 of an inch. The insert or slug is then placed within the copper cathode holder 22 which has a mating internal diameter of slightly greater than the inch external diameter of the slug 28. The extreme right hand part of the wide portion 26 of the cathode holder (which has been up to this point merely an extension of the cylindrical portion 26) is now swaged as indicated at 30 so as to secure the insert 28 within the cathode holder. The finished cathode assembly as shown in the drawing will then be attached to the cathode pin of the lamp of the type shown in either of the above mentioned US. Patents (and preferably of the type shOWn in FIGURE 1 of US. Patent No. 3,390,297), as more fully explained in both these patents.

Hollow cathode lamps (of the type shown in FIGURE 1 of said US. Patent No. 3,390,297) in which the cathode assembly is as shown in the instant drawing and as described herein above have been successfully tested and are currently being manufactured for commercial utilization. It has been found that neon is the preferable fill gas for the finished lamp, since it yields higher intensities for both the tellurium spectral radiation (as measured at the 2143 A. line for tellurium) and for tin (at both the 2246 A. and the 2863 A. tin lines) than the other conventional fill gas, argon. A neon fill gas pressure of approximately 8 torr has proved highly satisfactory. Lamps according to the invention have been successfully run for over 200 hours at 30 milliamps (6 ampere hours). Lamps manufactured using the cathode assembly according to the invention were operable satisfactorily at higher currents (30 milliamps) than previous lamps'using an internal coating or insert (28) in the interior of the cathode holder of pure tellurium (about 18 milliamps being the maximum practical with such prior lamps). Thus, the reduced vapor pressure and increased melting temperature of the tintelluride insert yielded improved operating characteristics, in addition to simplifying manufacture and lowering the risk of cracking during use (because of the improved ductility of tin-telluride compared to the brittle tellurium). A further advantage is the dual purpose nature of the finished lamp, lamps according to the invention actually having higher tin spectral radiation intensity than previous lamps, in which the interior of the cathode is molten tin (during operation).

Thus, applicant has provided an improvement in the cathode assembly for a spectral lamp, having improved stability, greater intensity, simpler manufacture, and dual utility (for both tellurium and tin spectral radiation). Thus, the invention accomplishes the purposes desired, although only relatively simple steps are required to manufacture the improved hollow cathode assembly. Although a specific exemplary embodiment of the invention has been fully disclosed, the invention itself is defined by the scope of the appended claims.

What is claimed is:

1. In a hollow cathode assembly for a spectral radiation lamp, said hollow cathode assembly being of the type comprising a substantially cup-shaped holder the interior hollow surface of which comprises a metallic substance for which spectral radiation is desired, the improvement in which:

said metallic substance comprises tin telluride, consisting of about 48% tin and about 52% tellurium by weight;

whereby an improved bright hollow cathode source is obtained for both tellurium spectral radiation and tin spectral radiation.

2. An improved cathode assembly, according to claim 1 in which said cup-shaped hollow cathode holder constitutes substantially pure copper.

3. An improved cathode assembly, according to claim 1, in which said metallic substance comprises a generally cylindrical insert having a generally cylindrical central aperture, said insert being positioned within the interior of said cup-shaped holder.

References Cited UNITED STATES PATENTS 3,183,393 5/1965 Paterson 313311 X 3,328,623 6/1967 Hale et a1 313-311 3,374,382 3/1968 Young 313-311 X JOHN W. HUCKERT, Primary Examiner ANDREW J. JAMES, Assistant Examiner US. Cl. X.R. 3 133 11 

